Biological Catalysis of Respiratory Oxygen Reduction

The superfamily comprises three classes cytochrome c oxidases (CcOs) , quinol oxidases, and cytochrome cbb3 oxidases CcOs are the only HCOs present in eukaryotes. Enzymes of all classes contain a binuclear heme-Cu catalytic site and all act as proton pumps. The water-soluble electron carrier, ferrocytochrome c, is the physiological electron donor for CcOs and cytochrome cbb3 oxidases. Certain CcOs can also oxidize specific high-potential iron-sulfur proteins (HiPIPs). Quinol oxidases catalyze a 2e/2H+ oxidation of various quinols to quinones as the source of electrons for O2 reduction. 

All HCOs also have a six-coordinate heme in the same subunit as the catalytic site. In addition aU cytochrome c oxidases have a binuclear Cua site and some also have a six-coordinate heme in the noncatalytic subunit (subunit n). These centers are absent in quinol oxidases. The six-coordinate heme(s) and Cua sites are responsible for electron-relay between the external electron 

With its four redox centers (Cua, heme a, heme as and Cub) mammalian CcO can exist in one of four redox states fully oxidized and singly reduced, which are aerobically stable and mixed-valence (2e-reduced), 3e , and fully reduced, all of which reduce O2 to the redox level of water within 200 //s of O2 binding This rate is substantially faster than the average rate of electron 

At physiologically relevant potentials with Rn(NFl3)g in air-saturated buffers, turnover frequency of up to 500 s was observed.  

Cytochrome ebbs oxidases comprise the third class of enzymes belonging to the HCO superfamily Like other HCOs, cytochromes ebbs catalyze reduction of O2 at a bimetallic heme/Cu site, contain an additional six-coordinate heme in the catalytic subunit, and act as proton pumps. Although both CcOs and cytochromes ebbs utilize cytochrome c as the physiological reductant, structurally and functionally they differ more than CcOs and quinol oxidases do. Unlike CcOs, which are distributed throughout all three domains of life, cytochromes ebbs are found almost exclusively in Proteobacteria (a group of Gram-negative 

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